A resin tube is well known as the duct having such the duct wall as described above, and an example of conventionally known fluid concentration measuring device which measures a concentration of a fluid such as a blood flowing in the resin tube is described in Patent Document 1. The fluid concentration measuring device described in the Patent Document 1 comprises: a transmitted light intensity measurement unit having a light source which supplies light into the duct from a light supply part on a surface of the duct; and a light receiving element which receives the light, which has been passed from the light supply part through the duct wall and the blood inside the duct, at a light receiving part located on an opposite side in a diametrical direction of the duct relative to the light supply part, and outputs a signal indicating an intensity of the light.
The fluid concentration measuring device described above has, further, a light path distance setting means which sets a plurality of light path distances between the light supply part and the light receiving part; and a fluid concentration output means which, from the light intensity at the light receiving part with regard to each of the plurality of light path distances, obtains a plurality of relational expressions, each of which indicates a relation between the light intensity and a blood concentration when the light from the light supply part is received by the light receiving part over each of the light path distances, based on the Beer-Lambert law, and obtains a concentration of the blood in the resin tube from the light intensity at the light receiving part based on the relational expressions for the plurality of light path distances and outputs the concentration of the blood.